Various types of catalytic hydrocarbon conversion reaction systems have found widespread utilization throughout the petroleum and petrochemical industries for effecting the conversion of hydrocarbons to different products. The reactions employed in such systems are either exothermic or endothermic and, of more importance to the present invention, often result in either the net production of hydrogen or the net consumption of hydrogen. Typical of the net hydrogen-producing hydrocarbon reaction systems are catalytic reforming, catalytic dehydrogenation of alkylaromatics, and catalytic dehydrogenation of paraffins. Commonly employed net hydrogen-consuming reaction systems are hydrotreating, hydrocracking, and catalytic hydrogenation. Of the above-mention net hydrogen-producing and consuming hydrocarbon reaction systems, catalytic reforming ranks as one of the most widely employed.
With the operation of net hydrogen producing processes the hydrogen balance for the petroleum refinery or petrochemical complex may result in excess hydrogen, i.e., the net hydrogen-producing reaction systems produce more hydrogen than is necessary for the net hydrogen-consuming reaction systems. In such an event, the excess hydrogen may be sent to the petroleum refinery or petrochemical complex fuel system. However, because the excess hydrogen often has admixed therewith valuable components such as C.sub.3 + hydrocarbons, it is frequently desirable to recover the three components by treating the excess hydrogen prior to its passage to fuel.
Because hydrogen is relatively expensive, it has become the practice within the art of hydrocarbon conversion to supply hydrogen from reaction systems which result in the net production of hydrogen to reaction systems which are net consumers of hydrogen. Occasionally, the net hydrogen being passed to the net hydrogen-consuming reactions systems must be of high purity due to the reaction conditions and/or the catalyst employed in the systems. Such a situation may require treatment of the hydrogen from the net hydrogen-producing reaction systems to remove hydrogen sulfide, light hydrocarbons, etc. from the net hydrogen stream.
It is well known that high quality petroleum products in the gasoline boiling range, including--for example--aromatic hydrocarbons such as benzene, toluene, and the xylenes, are produced by the catalytic reforming process wherein a naphtha fraction is passed to a reaction zone wherein it is contacted with a platinum-containing catalyst in the presence of hydrogen. By virtue of its wide application and its utilization as a primary source of hydrogen for the net hydrogen-consuming reactions systems, catalytic reforming has become well known in the art of hydrocarbon conversion reaction systems. Generally, the catalytic reforming reaction zone effluent comprising gasoline boiling range hydrocarbons and hydrogen is passed to a vapor-liquid equilibrium separation zone and is therein separated into a hydrogen-containing vapor phase and an unstabilized hydrocarbon liquid phase. A portion of the hydrogen-containing vapor phase may be recycled to the reaction zone. The remaining hydrogen-containing vapor phase is available for use either by the net hydrogen-consuming processes or as fuel for the petroleum refinery or petrochemical complex fuel system. While a considerable portion of the hydrogen-containing vapor phase is required for recycle purposes, a substantial net excess is available for the other uses.
Because the dehydrogenation of naphthenic hydrocarbons is one of the predominant reactions of the reforming process, substantial amounts of hydrogen are generated within the catalytic reforming reaction zone. Accordingly, a net excess of hydrogen is available for use as fuel or for use in a net hydrogen-consuming process such as the hydrotreating of sulfur-containing petroleum feedstocks. However, catalytic reforming also involves a hydrocracking function among the products of which are relatively low molecular weight hydrocarbons including methane, ethane, propane, butanes and the pentanes; substantial amounts of which appear in the hydrogen-containing vapor phase separated from the reforming reaction zone effluent. These normally gaseous hydrocarbons have the effect of lowering the hydrogen purity of the hydrogen-containing vapor phase to the extent that purification is often required before the hydrogen is suitable for other uses. Moreover, if the net excess hydrogen is intended for use as fuel in the refinery or petrochemical complex fuel system, it is frequently desirable to maximize the recovery of C.sub.3 + hydrocarbons which are valuable as feedstock for other processes.
Many processes for the purification of hydrogen-rich gas streams from the effluent of hydrocarbon conversion reaction zones are disclosed. Well known processing procedures for hydrogen-containing effluent streams include steps to remove potentially valuable C.sub.3 + product hydrocarbons from hydrogen-containing gas streams. U.S. Pat. No. 5,238,555, issued Aug. 24, 1993, discloses a process wherein the effluent from a reforming zone is separated into a hydrogen containing stream and a hydrocarbon effluent. Both streams are chilled and the chilled streams are contacted to absorb lighter hydrocarbons from the hydrogen-containing stream. U.S. Pat. No. 5,178,751 shows another arrangement for separating light hydrocarbons from a chilled hydrogen-containing stream recovered from a reforming zone using a chilled hydrogen stream. In U.S. Pat. No. 5,178,751 the hydrogen-containing stream and the hydrocarbon steam are admixed and chilled together in a single chiller, but the admixture of the streams precludes any independent control of the temperature of the gas and liquid phases.